The first use of computers in the study of disputed authorship study was probably Alvar Ellegård's study of the Junius letters. Ellegård, professor of English at the University of Gothenberg in Sweden, did not use a computer to make the word counts, but did use machine calculations which helped him get an overall picture of the vocabulary from hand counts.
Ellegård, A. A Statistical Method for Determining Authorship: The Junius Letters 1769–1772. Gothenburg: Gothenburg Studies in English, 1962.
A Companion to Digital Humanities, ed. Susan Schreibman, Ray Siemens, John Unsworth. Oxford: Blackwell, 2004.